1. Field of the Invention
This invention relates to relay devices, and more particularly to a motor starting relay device utilizing a positive temperature characteristics of positive temperature coefficient (PTC) resistance elements such as positive characteristic thermistors.
2. Discussion of the Prior Art
A relay device operating on the resistance temperature characteristic of a positive characteristic (PTC) thermistor has been extensively employed for starting an electric motor such as a compressor motor for a refrigerator or an air conditioner. The motor-starting relay device is provided for operating the starter circuit of an electric motor of capacitor start type or split-phase start type; that is, it operates as a kind of off-delay relay.
As disclosed by Post-examined Japanese Utility Model Publication (Kokoku) Sho-58-34722/(1983), Post-examined Japanese Patent Publication (Kokoku) Sho-63-18817/(1988), and Unexamined Japanese Model Application (Kokai) Hei-2-2802/(1990), a conventional motor-starting relay device uses one PTC thermistor.
A conventional split-phase start type single-phase motor is shown in FIG. 10, in which the PTC thermistor 101 is connected to the auxiliary winding 104 of the motor 102. For an initial period of time immediately after the power switch 106 is turned on to start the motor, the temperature of the PTC thermistor 101 is low, thus operating as a low-resistance element of several tens of ohms (.OMEGA.). Since the impedance of the auxiliary winding circuit which is determined from the low resistance of the PTC thermistor and the inductance of the auxiliary winding 104 is different from the impedance of the main winding circuit which is determined from the inductance of the main winding 103, the current I2 in the auxiliary winding 104 is shifted in phase to the current I1 in the main winding 103 of the motor 102. This phase difference gives rise to a rotating magnetic field, as a result of which the armature of the motor starts rotation. After the motor has been started, the PTC thermistor 101 generates heat by itself with the lapse of time; that is, its temperature is increased, and accordingly its resistance is greatly increased (to several tens of kilo-ohms (k.OMEGA.) to several hundreds of kilo-ohms (k.OMEGA.)) so that the current I2 flowing in the auxiliary winding 103 is greatly decreased to a micro current (several mili-ampere (mA)) only. Thus, the motor 102 is allowed to rotate in steady state. As is apparent from the above description, the PTC thermistor 101 operates as a starting resistor low in resistance, and as a high resistance element which to decrease the current I2 flowing in the auxiliary winding 104 is greatly decreased into slight current. That is, the PCT functions as a relay device which substantially electrically disconnects the auxiliary winding.
As was described above, in the conventional PTC relay device, the PTC thermistor is used as a high resistance element to substantially electrically disconnects the auxiliary winding. Therefore, during the steady operation of the motor, it is essential for the PTC thermistor to generate heat thereby to be maintained high in resistance. As a result, the PTC thermistor may consume electric power up to 3 to 4 watts.
On the other hand, it is necessary for the PTC thermistor to show high resistance for the relaying operation, and to meet the requirement that it should operates as a starting resistor low in resistance. Hence, in the prior art using only one PTC thermistor, the resistance temperature characteristic required for the PTC thermistor is determined, and therefore it is difficult to reduce the power consumption of the relay device.